The present invention relates to polyisocyanates of the formula (I)
OCNxe2x80x94R1xe2x80x94[xe2x80x94NXxe2x80x94COxe2x80x94Oxe2x80x94R2xe2x80x94Oxe2x80x94COxe2x80x94NXxe2x80x94R1xe2x80x94]nxe2x80x94NCOxe2x80x83xe2x80x83(I) 
in which the units R1, X and R2 have the following meanings:
R1 is a unit of the formula (II)
xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94xe2x80x83xe2x80x83(II), 
X is hydrogen or a unit of the formula (III)
xe2x80x94COxe2x80x94NHxe2x80x94R1xe2x80x94NCOxe2x80x83xe2x80x83(III) 
with the proviso that at least one of the units X in the polyisocyanates of the formula I is a unit of the formula III; and
R2 
is a 4-, 5- or 6-membered cycloalkylene radical in which up to 4 hydrogen atoms may be substituted by C1 to C4 alkyl radicals and one or two methylene units may be substituted by an oxygen atom, a sulfur atom and/or a tertiary nitrogen atom which additionally carries a C1 to C4 alkyl radical, or
is a C2 to C4 alkylene radical in which inserted between 2 carbon atoms or between one carbon atom and a hydrogen atom there is a 4-, 5- or 6-membered cycloalkylene radical in which up to 4 hydrogen atoms may be substituted by C1 to C4 alkyl radicals and one or two methylene units may be substituted by an oxygen atom, a sulfur atom and/or a tertiary nitrogen atom which additionally carries a C1 to C4 alkyl radical,
and the index n is 1, 2 or 3.
The invention additionally relates to isocyanate mixtures and to 2-component coating compositions comprising these isocyanates; to coating processes in which these 2-component coating compositions are used; and to the coated articles produced by these processes. Crosslinking polyisocyanates and binder components having isocyanate-reactive groups are widely known in the form of, for example, 2-component (2 K) coating materials (cf. Kunststoff Handbuch, Volume 7, Polyurethane, 2nd edition, 1993, Carl Hanser Verlag Munich Vienna, pp. 599 to 642, and M. Bock, Polyurethane Fxc3xcr Lacke und Beschichtungen, Vincentz-Verlag, Hannover, 1999). These 2-component coating compositions comprise as binder, for example, a polymeric polyol and as crosslinker component (hardener) a compound having two or more free isocyanate groups.
The service properties of coating materials whose polymeric binders have been crosslinked with the isocyanates are much better than those systems comprising low molecular mass binders. This relates in particular to service properties such as
insensitivity to mechanical exposure such as tension, strain, impact or abrasion;
resistance to moisture (in the form, for example, of water vapor) and dilute chemicals;
resistance to environmental influences such as temperature fluctuations and UV radiation; and
high gloss of the coated surfaces.
The hardeners (gelling agents) are expected not only to give the cured coatings the abovementioned service properties but also to improve, or at least have very little adverse effect on, the processing properties of the coating materials prior to their application.
So that the coating materials can be applied without problems by customary techniques, for example, by spraying them onto the surface to be coated, the coating materials are intended to possess a limited viscosity. Consequently, 2-component coating materials normally comprise solvents. The high solvent content of these coating materials itself causes problems however, since the coating-material processors have to take complex measures to avoid atmospheric emission of the solvents released when the coating materials are applied and dried. The search has therefore been on for hardeners which do not greatly increase the viscosity of the binder-containing component or which, even better, reduce it. These hardeners themselves of course must not display any noticeable volatility at room temperature, as is the case with customary commercial monomeric isocyanates such as hexamethylene diisocyanate or isophorone diisocyanate.
In addition, following their application, the 2-component coating compositions should cure as rapidly as possible so that the coated articles, following the application, may be subjected rapidly to further processing or use.
Polyisocyanates containing allophanate and biuret groups are known, for example, from EP-A-496 208, 524 501, 566 037 and WO 99/36455, where they are recommended for use as hardeners in 2-component coating systems. Suitable structural components include aliphatic monoalcohols and all customary isocyanates.
EP-303 150 discloses polyisocyanates containing allophanate groups, which are derived inter alia from diisocyanates and aliphatic alkanediols. The structural components from the group of the cyclic alkanediols and hexamethylene diisocyanate are present in these documents only in the context of lists of the suitable isocyanate and alcohol components. As evident from the experimental section of these documents, the focus is on polyisocyanates containing allophanate and biuret groups which are derived from noncyclic alcohols.
The allophanates derived from these alcohols have the disadvantage, in particular, that the 2 K coating systems produced with them are relatively slow to cure and even following complete curing still have a level of surface hardness which is inadequate for many applications.
Although the properties of the hardeners disclosed therein satisfy in most respects the customary requirements imposed on the processing properties of the uncured liquid coating systems which comprise these hardeners, and on the service properties of the coatings produced with the coating materials, these hardeners still appear in need of improvement with regard to the viscosity of the coating systems, their rate of cure, and the hardness of the coatings produced with them.
It is an object of the invention to provide this improvement.
We have found that this object is achieved by the compounds of the formula (I) defined at the outset, by mixtures and 2-component coating compositions comprising these compounds, and by articles coated with these 2-component coating compositions. The polyisocyanates of the formula (I) according to the invention are derived preferably from alcohols such as 1,3- or 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 1,3-cyclopentanediol or tetramethyl-1,3-cyclobutanediol.
Among these, particular preference is given to those wherein
n=1 and both units X are units of the formula III (diisocyanates Ia), or
n=1, one of the two units X is hydrogen and the other unit X is a unit of the formula III (diisocyanates Ib).
The polyisocyanates of the invention are normally prepared by
(i) reacting hexamethylene diisocyanate with a
divalent 4-, 5- or 6-membered cycloaliphatic alcohol in which up to 4 hydrogen atoms attached to a carbon atom may be substituted by C1 to C4 alkyl radicals and one or two methylene units may be substituted by an oxygen atom, a sulfur atom and/or a tertiary nitrogen atom which additionally carries a C1 to C4 alkyl radical, or
divalent C2 to C4 alkanediol in which inserted between 2 carbon atoms or between one carbon atom and a hydrogen atom there is a 4-, 5- or 6-membered cycloalkylene radical in which up to 4 hydrogen atoms may be substituted by C1 to C4 alkyl radicals and one or two methylene units may be substituted by an oxygen atom, a sulfur atom and/or a tertiary nitrogen atom which additionally carries a C1 to C4 alkyl radical,
the molar ratio of hexamethylene diisocyanate to the abovementioned alkanediols being from 50:1 to 3:1, preferably from 30:1 to 5:1, in the presence or absence of a catalyst which accelerates the formation of allophanates and urethanes,
(ii) deactivating the catalyst, if present, and
(iii) removing any unreacted isocyanate.
The reaction may be performed, for example, in the manner described in EP-A-524 501.
Examples of the catalysts used are quaternary ammonium carboxylates, such as N,N,N-trimethyl-N-(2-hydroxypropyl)ammonium 2-ethylhexanoate, quaternary ammonium hydroxides, or organotin or organozinc compounds.
The reaction is generally conducted at temperatures from 50 to 180xc2x0 C.
The reaction is generally continued until from 3 to 8 times the molar amount of isocyanate groups corresponding to the molar amount of divalent alcohol originally present in the reaction mixture have reacted. The progress of the reaction may be followed, for example, in accordance with customary titrimetric methods by measuring the NCO content of the reaction mixture.
The reaction is ended by cooling the reaction mixture and, if a catalyst has been used, by removing it, subjecting it to thermal decomposition, or adding a suitable agent which deactivates the catalyst. Examples of such agents are acids such as p-toluenesulfonic acid or dialkyl phosphates, e.g., dibutyl phosphate or di(2-ethylhexyl) phosphate.
After the end of the reaction, unreacted hexamethylene diisocyanate is generally removed by distillation, preferably down to a level of less than 0.5% by weight.
The reaction product, which is essentially free from hexamethylene diisocyanate, has a viscosity at room temperature of generally from 500 to 20,000 mPas, preferably from 1000 to 15,000 mPas measured in accordance with ISO 3219.
This procedure normally produces mixtures (M) comprising
polyisocyanates I,
urethanes of the formula IV (urethane IV)
OCNxe2x80x94R1xe2x80x94[xe2x80x94NHxe2x80x94COxe2x80x94Oxe2x80x94R2xe2x80x94Oxe2x80x94COxe2x80x94NHxe2x80x94R1xe2x80x94]pxe2x80x94NCOxe2x80x83xe2x80x83IV 
in which
the units R1 and R2 are as defined for formula I
and p is a number from 1 to 3,
the polyisocyanurates derived from hexamethylene diisocyanate.
Preferably, the fraction of polyisocyanates I is at least 10% by weight.
Particular preference is given to mixtures (M) comprising
from 20 to 80% by weight of polyisocyanates I,
from 0 to 50% by weight of urethanes IV, and
from 0 to 80% by weight of polyisocyanurates derived from hexamethylene diisocyanate.
The mixtures (M) may further comprise products of higher molecular mass, which come about as a result of the reaction of the polyisocyanates I with further hexamethylene diisocyanate or with the other types of molecule present in the mixture (M), to form uretdione groups, biuret groups, or isocyanurate groups. In general, the mixtures (M) contain such products in amounts of up to 20% by weight.
If desired, the polyisocyanates I of the invention may easily be separated from these mixtures by means of known separation methods such as gel permeation chromatography or high-performance liquid chromatography. Generally, however, this is unnecessary insofar as the polyisocyanates I are used in the form of the abovementioned mixtures as crosslinkers in 2-component polyurethane coating compositions.
The polyisocyanates I and the abovementioned mixtures (M) are particularly suitable as component B in the preparation of 2-component coating materials which as component A comprise a compound which carries polyisocyanate-reactive groups, preferably a hydroxy-functional polymer (A).
Said hydroxy-functional polymers (A) are, for example, polymers having a hydroxyl group content of from 0.1 to 20, preferably from 0.5 to 10% by weight. The number-average molecular weight Mn of the polymers is preferably from 1000 to 100,000, with particular preference 2000 to 10,000. The polymers concerned are preferably those which consist of more than 50% by weight of C1-C20 alkyl (meth)acrylate, vinylaromatic compounds having up to 20 carbon atoms, vinyl esters of carboxylic acids containing up to 20 carbon atoms, vinyl halides, nonaromatic hydrocarbons having 4 to 8 carbon atoms and 1 or 2 double bonds, unsaturated nitriles, and mixtures thereof. Particular preference is given to those polymers which consist of more than 60% by weight of C1-C10 alkyl (meth)acrylates, styrene, or mixtures thereof.
The polymers (A) further comprise hydroxy-functional monomers in accordance with the above hydroxyl group content and, if desired, further monomers, examples being ethylenically unsaturated acids, especially carboxylic acids, acid anhydrides or acid amides.
Examples of further polymers (A) are polyesterols, as are obtainable by condensing polycarboxylic acids, especially dicarboxylic acids, with polyols, especially diols.
Further suitable polymers (A) are polyetherols, which are prepared by addition reaction of ethylene oxide, propylene oxide or butylene oxide onto H-active components. Polycondensates of butanediol are also suitable.
The polymers (A) may of course also comprise compounds having primary or secondary amino groups.
Mention may be made, for example, of products known as Jeffamines, i.e., amino-terminated polyetherols, or oxazolidines.
In addition to the abovementioned components A and B, the 2-component coating compositions may further comprise other polyisocyanates and compounds having polyisocyanate-reactive groups, as are normally present in two-component coating materials.
The molar ratio formed from the sum of the isocyanate groups in the components B to the sum of the isocyanate-reactive groups of component (A) is preferably from 0.6:1 to 1.4:1, more preferably from 0.7:1 to 1.3:1, with very particular preference 1:1.
The coating materials of the invention may further comprise organic solvents, examples being xylene, butyl acetate, methyl isobutyl ketone, methoxypropyl acetate, and N-methylpyrrolidone. Solvents are used to establish the low viscosity which is desired for the processing of the coating material, i.e., for its application to substrates.
The coating materials may of course comprise further additives customary in coatings technology, examples being pigments, fillers, leveling assistants, etc.
They may further comprise catalysts for urethane formation, e.g., dibutyltin dilaurate.
The two-component polyurethane coating compositions may be prepared in conventional manner. Commonly, components A and B are mixed before the coating compositions are applied to a substrate. Mixing takes place normally from 0 to 12 h before application. The desired viscosity may be established using solvent.
The polyurethane coating compositions may be applied two-dimensionally to substrates in conventional manner by spraying, pouring, rolling, brushing, knife-coating, etc.
The coating compositions are particularly suitable for workpieces with surfaces of metal, plastic, wood, woodbase materials, ceramic, or glass.
The articles coated by these processes feature a surface which has very good mechanical propertiesxe2x80x94in particular, good hardness, flexibility, and chemical resistance.